As a hybrid drive system, Japanese Patent Laid-Open Publication No. 2000-343965 discloses a hybrid drive system which is provided with a path to transmit an engine output via a transmission to wheels and a path to input a rotating electric machine output to the transmission.
Also, Japanese Patent Laid-Open Publication No. 2001-173479 discloses a hybrid drive system which is provided with a drive motor to drive a vehicle and an engine to drive an electric generator, and compensates for torque variation with the drive motor when the engine is started by the electric generator.
In the system according to the Japanese Patent Laid-Open Publication No. 2000-343965, when the vehicle travels by only the engine output, the operation of the rotating electric machine is maintained in a stopped state, but the rotating electric machine remains to be connected to the drive system, and thus, the inertial mass and friction of the rotating electric machine cause a drive loss, which consequently inhibits the improvement of fuel economy of the engine.